fix(args): skip Megatron validate_args for external SGLang server

[surega21]

Summary

• Add is_external_model_server(args) predicate that returns True when --sglang-router-ip is set and --rollout-num-gpus == 0.
• Gate megatron_validate_args (and the associated varlen / pipeline-parallel rewrites) on not is_external_model_server(args).

Why

When rollout runs against an external SGLang model server, no local rollout GPUs are allocated. megatron_validate_args reads CUDA device properties during parse_args, which crashes on CPU-only pods that have no NVIDIA driver — this fails eval jobs before any rollout starts.

Skipping the Megatron block for external-server mode is safe because local Megatron isn't initialized in that path. miles_validate_args and sglang_validate_args still run for external-server evals.

Assumptions

• External-server mode is identified by the combination of sglang_router_ip is not None and rollout_num_gpus == 0. Setting only one of the two keeps the existing validation path.
• The skipped block (Megatron validation, variable_seq_lengths = True, allgather → alltoall rewrite, and the pipeline_model_parallel_size == 1 decoder assertion) only applies when local Megatron is initialized, which does not happen for external-server rollouts.

Risks

Low. Regular training and colocated-eval runs (both sglang_router_ip is None or rollout_num_gpus > 0) hit the unchanged code path.

Validation

• python3 train.py --sglang-router-ip <remote> --rollout-num-gpus 0 ... no longer trips megatron_validate_args at parse_args time.
• Regular Megatron training runs unaffected (predicate returns False).
• pre-commit run --files miles/utils/arguments.py clean.

[gemini-code-assist]

Code Review

This pull request introduces the is_external_model_server helper function to determine if the rollout runs against an external SGLang model server, allowing Megatron argument validation to be skipped on CPU-only pods. The review feedback suggests using getattr to safely access rollout_num_gpus to prevent potential AttributeErrors when args is a mock or custom namespace.

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